The invention relates to the display of piloting assistance information in the cockpit of an aircraft. Modern aircraft, in particular transport aircraft, generally include a system for displaying piloting assistance information in their cockpit. A system of this kind, for example of CDS (control and display system) type, controls the display of information on screens, termed head-down screens, in the cockpit: to view this information, the user, generally a pilot or copilot of the aircraft, must lower his or her head to look at the screens. They are therefore not able to see at the same time the surroundings of the aircraft through a windscreen of the cockpit. In order to allow the user to see an information display without having to lower his or her head to look at the screens, some aircraft are now provided with a display device configured to be secured to the head of the user. A device of this kind is commonly termed an HMD (head-mounted display). It is sometimes also termed an HWD (head-worn display). It generally includes a display securely mounted on goggles or on a helmet so that the user can see information displayed on the display when wearing the goggles or the helmet. In the remainder of the description, the term HMD refers equally to an HMD device and an HWD device. HMD devices including a helmet are used in military aircraft to assist in operations of firing on a target, for example. HMD devices including goggles are used for displaying piloting information, for example. The display is preferably transparent so that the user can view the information displayed in augmented reality superimposed on the environment. As illustrated in FIG. 4a, the information displayed on the HMD device 20, which is secured to the head 50 of the user, is generally displayed with a collimation distance corresponding to infinity. In practice, a collimation distance that is greater than a value of the order of 4 to 6 meters can be likened to an infinite collimation distance. The HMD device 20 is arranged to display the information such that it is perceived by the user as being displayed on a virtual surface 100 located at the collimation distance. Using a collimation distance corresponding to infinity allows the user to see this information clearly when he or she looks outside the aircraft, in particular through a windscreen 40 of the cockpit. However, as illustrated by FIG. 4b, when the direction 52 in which the user looks corresponds to an element 42 of an interior portion of the cockpit 3, which element is located at a distance D from the head 50 of the user, the user must strain to accommodate his or her eyes depending on whether he or she wants to look at this element 42 of the interior portion of the cockpit or the information displayed on the HMD device. In the present description, a distance with respect to the head of the user more specifically refers to a distance with respect to the eyes of the user. In particular, this distance may be defined with respect to a point that is equidistant from both eyes, located on a straight line passing through both eyes. Moreover, it is difficult for the user to clearly see both the element 42 of the interior portion of the cockpit and the information displayed by the HMD device.